A bloke in Unanderra put it to me the way most people do. He had a paved area under a patio roof off the back, the family never used it between May and September because the southerly came straight through, and he wanted to know what it would cost to "just put glass around it". His word was sunroom. He was expecting a fraction of what an extension costs, and he had every reason to expect that, because from the outside the roof and the slab are already there.
We spent about twenty minutes on his patio and then I had to give him an answer he wasn't after: everything visible was already built, and almost none of it could stay. The slab fell away from the house at the rate you'd want for water and completely the wrong rate for a floor. The roof was a single skin of sheeting with no insulation and no ceiling. And the wall he wanted to open up was carrying the roof of the house.
None of that means it was a bad idea. He's got the room now and it's the best part of the house. It just wasn't the job he thought he was buying, and the gap between those two things is what this is about.
Outdoor, enclosed, or a room
There are really three different things people mean by closing in a patio, and they cost wildly different amounts.
The first is keeping it outdoors and making it usable more of the year. Café blinds, outdoor roller screens, or a decent windbreak on the side the weather comes from. It stays an outdoor space, nothing about its approval or its construction changes, and for a lot of families it genuinely solves the problem. If the complaint is wind and driven rain rather than temperature, start here, because the cheapest version is often the right one.
The second is an enclosed sunroom that isn't part of the conditioned house. Glazed or screened in, a door between it and the living room that mostly stays shut, no heating or cooling, still fairly close to the outside temperature. Useful in spring and autumn, honest about what it is.
The third is a habitable room: insulated, sealed, heated and cooled, open to the rest of the house, the sort of space you'd put a lounge in through winter. That is an extension. It gets built to the standards for a habitable room in the National Construction Code and it gets priced accordingly.
The mistake worth avoiding is paying for the third one and getting the second. That happens when the glass goes in but the slab, the roof and the wall are left as they were, and the room ends up cold in July and unbearable in January.
The slab is usually the surprise
An outdoor slab and a floor are not the same thing, and this is where most of the unexpected money goes.
A paved or concreted outdoor area is laid with a deliberate fall so water runs off it. Bring that inside and you have a floor that slopes, which you'll feel underfoot within a week and which makes joinery and skirtings awkward for as long as you own the house. Fixing it means a topping screed to level it, or in a worse case taking it out.
Then there's what's under it. A slab poured for a patio is often thinner than a house slab, it may not carry the reinforcement a room needs, and it frequently has no damp-proof membrane beneath it, because nobody needed one when the space was open to the air. Put a sealed floor covering over a slab with no membrane and moisture that used to evaporate freely has nowhere to go. It comes up through the slab, and it will lift flooring and mark the base of walls.
You can't tell any of this by looking. A core sample or a look at whatever paperwork exists for the original patio will tell you, and it's worth knowing before you price anything, because it's the difference between a straightforward job and a much bigger one.
The roof was built for shelter, not for a room
A patio roof is normally a single skin of sheeting on a fairly flat pitch, with no ceiling under it and nothing in between. As a shelter that's exactly right. As the roof of a room it has two problems.
The first is heat. Bare sheeting radiates the day's sun straight down at you in summer, and gives it all back to the sky at night in winter. The second is condensation. Once the space below is warm and holds moisture from people, cooking or a dryer, the underside of a cold uninsulated sheet is where that moisture turns back into water. It drips, and it drips onto the new plasterboard.
The fix is insulation and a ceiling, sometimes an insulated roof panel instead of the sheeting, and enough fall to still shed water once you've added the depth. That last part catches people, because a low-pitch patio roof that was set out to just clear the eaves doesn't always have room to grow. Where it doesn't, the roof comes off and gets rebuilt, which is when the budget for the sunroom stops looking like a sunroom.
If you're at the earlier stage of picking what to put over an outdoor space in the first place, the choices between open battens, sheeting and a verandah roof are worth understanding before you build anything, and I've gone through them in pergola, patio roof or verandah.
The wall that used to be outside
The last piece is the existing external wall. Enclose the patio and that wall becomes an internal one, which raises the question everybody eventually asks: can we take it out?
Sometimes. That wall is often carrying roof load, and on brick homes it's usually doing more than it looks like it's doing. Taking it out means a beam sized by an engineer, posts or piers to take that beam's load down to something solid, and the work to get it in without the roof moving. It's ordinary construction and we do it regularly, but it's engineered work with a real cost, not a demolition job.
There's a cheaper middle ground people overlook. Leaving the wall and widening the opening, or replacing a window with a door, gets you most of the connection to the new space for a fraction of the structural work. Whether that's enough depends on how you want to use the room, and it's a decision worth making with a builder standing in the house rather than from a plan.
Weatherproofing is the quieter half of this. The old external wall has flashings, weepholes and a damp-proof course that were all doing a job at the junction between outside and inside. Once there's a room on the other side, that junction moves, and the detailing has to move with it. Skipped, it shows up as damp at the base of the wall a couple of winters later.
Glass, and the reason a sunroom can be unusable
The whole appeal of a sunroom is glass. Glass is also the least thermally efficient thing in any wall, and a room made mostly of it behaves like it.
Aspect decides how much this bites. A north-facing sunroom with an eave or an awning over it is the good version: winter sun comes in low and warms it, summer sun is high and gets shaded out. A west-facing one with a large area of unshaded glass is the version people stop using, because from about two in the afternoon in summer it's a glasshouse, and no air conditioner is going to win that argument cheaply.
The levers are the same ones that decide how a new house performs: how much glass, which way it faces, what shades it, and what the glazing itself is. The Commonwealth explains the rating system those choices feed into at homeenergyrating.gov.au. Worth knowing too that in NSW, renovation work above a cost threshold needs a BASIX certificate of its own, so on a job of any size this stops being just a comfort question and becomes a compliance one. I've covered how that assessment actually works in the post on BASIX and the energy rating.
Approvals change when it stops being open
An open pergola or patio roof is one of the more common jobs that can go ahead as exempt development in NSW. Enclosing it usually ends that. Once the sides are closed and it reads as additional floor area rather than a shade structure, it's assessed as an addition to the house, which means a complying development certificate or a DA depending on your block.
That isn't a reason to avoid it, it's a reason to find out early, before you've bought glazing. The council approval guide for decks, pergolas and carports sets out the three paths, and NSW publishes its general guidance for homeowners at nsw.gov.au. Your council or a private certifier will confirm what applies at your address, usually for nothing at the enquiry stage.
So what does it cost
Once the slab, the roof, the wall and the glazing are all being done properly, you are building an extension with a head start, and it prices like one. A ground-floor extension in NSW generally runs around $3,500 to $6,000 per square metre, and an enclosed sunroom usually sits at the lower end of that, because there's no plumbing in it and often less new roof and footing work than a room built from scratch. What's already there does save you money. It just saves less than it looks like it should. There's a fuller breakdown of what moves that rate on our extension cost guide.
The version worth avoiding is the one where the glass goes in and nothing else does. It's cheap, it looks finished, and it produces a room that's too cold half the year and too hot the other half, which is exactly the room you were trying to fix.
If the space is genuinely worth having year-round, price it as a room from the start, and compare it honestly against simply building out into the yard, which sometimes wins. If it isn't, screens and blinds and honest expectations will serve you better than a sunroom that gets used twice.
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